The invention relates to image recording apparatuses for fusing toner images with a heat roll and, more particularly, to an image recording apparatus in which the surface temperature of the heat roll is controlled on two different levels, a sheet fusing level and a fusing standby level.
In electronic copying machines and image recording apparatuses, such as facsimile machines or laser printers using xerography, a latent electrostatic image formed on a photosensitive body is developed into a toner image by means of toner. This toner image is transferred onto a sheet or sheet member and fused thereafter. Among various techniques of fusing toner images, a technique based on a heat roll has been extensively used. The heat roll is so constructed that a heating body is contained inside the heat roll and the surface temperature of the heat roll is increased by conducting the heating body. The heat roll is used as a counterpart for a pressure roller which is in pressure contact with the heat roll. When the sheet passes between the heat roll and pressure roll, the toner is fused by the surface temperature of the heat roll and the fused toner is pressed onto the sheet surface to be fused and fixed.
In such conventional image recording apparatuses, an increase in the temperature of the heat roll to a toner image fusible temperature takes place abruptly and then such fusible temperature is maintained until the sheet arrives. However, such a technique has encountered the following problems.
(1) The relatively high fusible temperature of the heat roll leads to an increase in heat radiation and a waste of power. The heat roll whose surface temperature is high is dangerous if touched by a hand when no printing is performed, which is a safety problem. PA1 (2) Since the heat roll is heated to a constant temperature for a long period of time, such constant temperature must be such that other parts of the image recording apparatus are not affected thereby. This has been a constraint in setting the fusing temperature to a temperature slightly lower than the ideal temperature required for continuous fusing. In other words, there is a risk in the conventional image recording apparatuses that incomplete fusing may result under certain conditions.
To overcome these problems, image recording apparatuses which can control the surface temperature of their heat roll on two levels have been marketed.
FIGS. 7(a) to 7(c) show an exemplary temperature control of a laser printer using a polygonal mirror, which is an exemplary image recording apparatus of such type. In FIG. 7(a) shows the timing of driving a motor for rotating the polygonal mirror (hereinafter referred to as "ROS motor"). Upon arrival of a print command from a host computer to the laser printer at a timing T.sub.1, the ROS motor starts rotating. When the speed of the ROS motor reaches a desired constant speed after an interval t.sub.1 has elapsed, a main motor of the laser printer starts rotating at a timing T.sub.2 as shown in FIG. 7(b). This main motor serves to rotate not only the photosensitive body of the image recording apparatus but also its heat roll to get ready for fusing a sheet.
FIG. 7(c) shows an exemplary temperature control of the heat roll. The heat roll starts conduction at a timing at which a power supply of the image recording apparatus has been turned on and maintains a first set temperature S , which is higher than room temperature, once it has reached such temperature. As from the timing T.sub.2 at which the main motor has been activated, the heat roll is controlled so that its temperature is increased to a second set temperature S.sub.2 which is higher than the first set temperature S.sub.1.
Now, after the activation of the main motor, a latent electrostatic image is formed on the photosensitive body and developed into a toner image by toner, and the toner image is transferred onto a sheet. Thus, it is important that the heat roll has its surface temperature increased to the second set temperature S.sub.1, which is a predetermined fusing temperature, within an interval t.sub.2 from the timing T.sub.2 to the arrival of the front end of the sheet which interval is a finite period. An interval t.sub.3 from such arrival timing T.sub.3 is a period during which the sheet is being fused while passing through the heat roll. After an interval t.sub.4 from a timing T.sub.4 at which the fusing has been completed by the heat roll, the sheet is discharged; the main motor stops its operation; and the surface temperature of the heat roll starts decreasing to the first set temperature S.sub.1. Thereafter, the ROS motor is turned off when an interval t.sub.5 has elapsed. The rotation of the ROS motor is not stopped immediately because the ROS motor must check whether or not a next print command is being received.
FIG. 8 shows the above-described control more specifically. This image recording apparatus has a CPU (central processing unit) and the actual control is effected in accordance with a program stored in a storage medium such, as a ROM (read only memory), in such a manner as shown in FIG. 8.
Specifically, upon turning on of a main switch of the image recording apparatus, the CPU starts conduction of the heater contained within the heat roll so that the heater is subjected to a warmup for the first set temperature S.sub.1 (Step (1) in FIG. 8). On the side of the heat roll is a temperature detecting element, with which the CPU checks whether or not the detected temperature is equal to the first set temperature S.sub.1 (Step (2)). When the surface temperature of the heat roll reaches the first set temperature S.sub.1 (Y), at which the image recording apparatus gets ready to fuse, the CPU lights up a ready lamp on an operation panel (Step (3)).
The image recording apparatus enters a standby state under this condition and monitors a timing at which a print command arrives from the host computer (Step (4)). Upon arrival of the print command (Y), the CPU controls an ROS motor drive circuit to start driving the ROS motor (Step (5)). When the ROS motor has reached a predetermined speed (Step (6), Y) thereafter, i.e., when the interval t.sub.1 shown in FIG. 7 has elapsed, the driving of the main motor is started (Step (7)). Successively, the surface temperature of the heat roll is controlled so as to reach the second set temperature S.sub.2 which is a fusible temperature (Step (8)). Such control is continued until a series of print operations have been completed by fusing a sheet and discharging the sheet to a discharge tray (Step (9)).
Upon completion of the print operations (Step (9), Y), the driving of the main motor is stopped (Step (10)), and the surface temperature of the heat roll is reset to the first set temperature S.sub.1 (Step (11)). Thereafter, arrival of a next print command is monitored within the interval t.sub.5 (Steps (12), (13)). Upon arrival of the print command (Step (12), Y), the CPU returns to Step (7) to start driving the main motor. If no print command has arrived (Step (13), Y), the CPU stops driving the ROS motor (Step (14)). If, on the other hand, the print command has arrived at this stage (Step (15), Y), the CPU returns to step (5) and starts driving the ROS motor.
As described above, in the conventional image recording apparatus, the timing of changing the set temperature of the heat roll from the first set temperature S.sub.1 to the second set temperature S.sub.2 for fusing coincides with the main motor driving start timing (FIG. 7). Therefore, upon the start of driving of the main motor, the heat roll is forced to begin heating up drastically to increase its surface temperature toward the second set temperature S.sub.2.
In the meantime, when the main motor has started rotating, not only formation of a latent electrostatic image on the photosensitive body is started, but also a sheet is fed from a sheet feed tray and arrives near the photosensitive body to cause a toner image to be transferred onto the sheet. After the transfer, this sheet is forwarded to the heat roll. The interval of time elapsed from the photosensitive body or heat roll drive start to the arrival of the front end of the sheet at the heat roll is the interval t.sub.2 shown in FIG. 7.
However, the interval t.sub.2 has, in effect, been reduced by the ever-increasing print or recording speed of image recording apparatuses, such as laser printers, achieved by recent technical improvements, together with a trend toward compact design. Such reduction in the interval t.sub.2 has caused, in some cases, a problem of inadequately fusing the toner image in a couple of starting pages introduced into the heat roll, although it depends on the heat roll material and the type of sheet to be fused.
FIG. 9 is a diagram for a description of such a case. In FIG. 9, the surface temperature of the heat roll is set to the first set temperature S up to the timing T.sub.2 and, from this timing T.sub.2 at which the main motor starts driving, the surface temperature is increased to the second set temperature S.sub.2. However, in the case shown in FIG. 9, the heat roll has not reached the second set temperature S.sub.2 until the fourth sheet has arrived. As a result, the fusing of the first to third sheets, among others, is incomplete.
The image recording apparatuses, such as laser printers, are usually used to print or record only one sheet or several sheets at a time. Thus, defective fusing of a first print or copy or in a couple of first prints or copies may often invite defects of many other following prints or copies, which is a serious problem.